Method of making a CMOS device with high and low voltage transistors

Abstract: A semiconductor device having high (76,88) and low voltage (80,84) transistors on the same chip. High voltage NMOS transistor (76) comprises a polysilicon gate (40) doped at a first dopant level. Low voltage NMOS transistor (80) comprises a polysilicon gate (44) doped at a second dopant level. The second dopant level is higher than the first. High voltage PMOS transistor (84) comprises a polysilicon gate (48) doped at a third dopant level. Low voltage PMOS (84) transistor comprises a polysilicon gate (52) doped at a fourth dopant level. The fourth dopant level is higher than the third. By allowing the electric field to penetrate into the bottom of the high voltage transistor gate, the effective oxide thickness is made greater.

Description: FIELD OF THE INVENTION This invention generally relates to semiconductor processing and more specifically to a method of forming a semiconductor device. BACKGROUND OF THE INVENTION As the density of integrated circuits increases, it becomes necessary to shrink the dimensions of NMOS and PMOS transistors. Proper scaling of NMOS and PMOS transistors typically requires that the operating voltage be decreased as the gate oxide thickness is shrunk. Otherwise, the electric field will become too large in the gate oxide and, consequently, the gate oxide will eventually fail. On the other hand, if the operating voltage is decreased, the device will no longer be compatible with most of the current packaged integrated circuits which operate at a standard voltage. For, instance, most circuits using CMOS transistors with gate lengths of 0.8 microns or more operate at 5.0V. When the gate length in decreased to 0.5 microns and the gate oxide thickness to 90-120 A (10A=1nm), the voltage is lowered to 3.3V in order to maintain reliability of the gate oxide. Thus, a device is needed that has input/output peripheral sections that operate at 5.0 V so that the device may be used in systems using other chips operating at 5.0 V while allowing other portions of the device to operate at 3.3 V. The same problem occurs when the gate length is reduced from 0.5 µm to .35 µm or .25 µm. At .35 µm, the voltage is reduced to 2.5V or lower in order to maintain the integrity of the gate oxide. One method that has been used to overcome this problem uses longer gate lengths in the input/output CMOS transistors in order to minimize the hot carrier stress problem. However, gate insulator reliability may still be a problem due to the large electric field in the gate insulator. Another method uses a thicker gate oxide for the input/output sections. This lowers the electric field in the high voltage CMOS transistors. However, this method requires a resist to be patterned on the gate oxide to remove the oxide from one portion of the chip and then strip the resist and grow the second gate oxide of a different thickness. As a result, defects and contamination may occur in the gate oxide. Another approach uses two polysilicon layers. One polysilicon layer is placed over a first gate of one thickness. Next, a second gate oxide is grown and another polysilicon layer is deposited over the second gate oxide. This process however, adds to many additional process steps. From US-A-4 764 478 a method of manufacturing a MOS transistor by dual species implantation and rapid annealing is known. Boron is introduced as a first impurity in the surface of a polycrystalline silicon layer. Arsenic or silicon, boron difluoride is ion-implanted as a second impurity having a greater mass than the first impurity, thereby ensuring the uniform redistribution of the first impurity in the polycrystalline silicon layer. Subsequent heat treatment is carried out at a low temperature and for a short time. EP-A-0 414 400 discloses a p-channel depletion device which is serially connected to an NMOS device. The depletion device comprises a silicon substrate, an n-well region, p + source and drain regions, and a p- polysilicon gate which is formed by counterdoping an n--gate with the normal p + source drain implant. From MOS Field-Effect Transistors and Integrated Circuits, P. Richman, Wiley 1973, Pages 220 - 221, it is known that PMOS load gates and devices are generally subjected to higher voltages than the corresponding NMOS devices. GB-A-2 016 801 discloses a reference voltage generating device which comprises two insulated gate field effect transistors whose gates are doped differently so that their Fermi energy levels are different. One of the transistors has an n + -type gate and the other a p + -type gate. SUMMARY OF THE INVENTION According to the present invention, there is provided a method as defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a cross-sectional view of a semiconductor device formed by the method of the invention; FIG. 2 is a cross-sectional view of semiconductor body used as the starting point for the fabrication of a semiconductor device in accordance with the invention; FIGs. 3a-d are cross-sectional views of a semiconductor device during various fabrication steps according to the invention; FIGs. 4a-b are cross-sectional views of lightly and heavily doped poly gates, respectively; FIGs. 5a-b are band diagrams over the active channel of PMOS transistors having lightly and heavily doped poly gates, respectively; FIGs. 6a-b are C-V diagrams of NMOS and PMOS transistors, respectively, having a gate oxide of 120 A; FIGs. 7a-b are C-V diagrams of NMOS and PMOS transistors, respectively, having poly gates doped once during the source/drain implant; FIGS. 8a-b are C-V diagrams of NMOS and PMOS transistors, respectively, having extra poly gate doping; and FIG. 9 is cross-sectional view of a different semiconductor device not forming part of the claimed invention. Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The preferred embodiment of the invention will be described in conjunction with CMOS transistors using n+ polysilicon for NMOS gates and p+ polysilicon for PMOS gates. It will be apparent to those skilled in the art that other processes and devices, such as CMOS, BiCMOS, and BiNMOS devices, can also realize the benefits of this invention. For high voltage performance and packing density, the layout and thickness dimensions of CMOS transistors need to be decreased. This usually results in a decreased operating voltage in order to avoid reliability problems caused by increasing the electric field in the gate insulator. Currently, however, most integrated circuits still need to interact with other circuits using a standard higher voltage. Thus, there is a need for higher voltage input/output sections and lower voltage sections on the same chip. There is also a need to place higher voltage non-volatile memory on the same chip with high density lower voltage CMOS. The preferred embodiment of the invention provides high and low voltage CMOS on the same chip by means of using lower doping of the polysilicon gates in the high voltage portions of the circuit. In the low doping areas, the electric field from the gate insulator penetrates into the lower portion of the gate and decreases the peak electric field in the gate insulator. In other words, the high voltage CMOS transistors will behave as if they had a thicker gate oxide. Accordingly, the operating voltage can be increased to a higher voltage (e.g., back to 5.0 V) without causing an electric field sufficient to cause gate insulator reliability problems. The preferred embodiment of the invention is shown in Figure 1. Substrate 14 of device 10 is shown as a p++ substrate. It will be apparent to those skilled in the art that other substrates may alternatively be used. Optional epitaxial layer 18 is p-type into which a patterned p-well 23 may be implanted and which will contain the high voltage NMOS transistor 76 and low voltage NMOS transistor 80. N-well 22 is located in epitaxial layer 18. Field insulating regions 26 may comprise oxide and serve to isolate transistors 76, 80, 84, and 88 from each other. Gate oxide regions 32 and sidewall oxides 64 insulate polysilicon gates 40, 44, 48, and 52. High voltage NMOS transistor 76 is located in p-well 23 and comprises gate 40 and source/drain regions 68. Transistor 76 may also comprise lightly doped drains (LDDs) 56. Low voltage NMOS transistor 80 is also located in p-well 23 and comprises gate 44 and source/drain regions 70 and gate insulator 32. Transistor 80 may also comprise LDDs 58. Low voltage PMOS transistor 84 and high voltage PMOS transistor 88 are located in n-well 22. Low voltage PMOS transistor 84 comprises a gate 48, gate insulator 32 source/drain regions 72, and optionally LDDs 60. High voltage PMOS transistor 88 comprises a gate 52, source/drain regions 74, and optionally LDDs 62. Figure 2 shows semiconductor body 12 after the formation of p-type epitaxial layer 18, n-well 22, field insulating regions 26, and gate oxide 32. The preferred method for forming device 10 in the structure of Figure 2 will now be described with reference to Figures 3a - d. Referring to Figure 3a, a layer of polysilicon 36 is deposited over the gate oxide 32 covering the surface of semiconductor body 12. Polysilicon layer 36 may have a thickness in the range of .2 to .4 microns, preferably .25 microns. For the low voltage transistors, the thickness of polysilicon layer 36 is determined by the depth of the source/drain junctions. Polysilicon layer 36 may be thicker than the source/drain junctions so that when the source/drain junctions are implanted, the dopant does not extend to the bottom of the polysilicon gates. Masking layer 38, for example photoresist, is deposited on the surface of polysilicon layer 36 as shown in Figure 3a. Masking layer 38 masks portions of polysilicon layer 36 where high voltage NMOS transistor 76 and both high and low voltage PMOS transistors 84 and 88 are to be formed. The exposed portion of polysilicon layer 36 is implanted with phosphorus (e.g., 5E15 P/cm 2 ) and annealed. Typically, the implant will be a high energy implant in the range of 1E15 to 1E16 P or As/cm 2 . Masking layer 38 is then removed and a second masking layer 39 is deposited, as shown in Figure 3b. Masking layer 39 masks portions of polysilicon layer 36 where both high and low voltage NMOS transistors 76 and 80 and high voltage PMOS transistor 88 are to be formed. The exposed portion of polysilicon layer 36 is then implanted with boron (e.g., 7E15 B/cm 2 ) and annealed. Typically, the implant will be a high energy implant in the range of 1E15 B/cm 2 to 1E16 B/cm 2 . An anneal is used to anneal implant damage before gate etch and to drive both the n-type and p-type dopant material to the bottom of polysilicon layer 36. Masking layer 39 is then removed. Referring to Figure 3c, polysilicon layer 36 is then patterned by microlithography and etched, along with gate oxide 32 to form gates 40, 44, 48, and 52. At this point, LDDs 56, 58, 60, and 62 may optionally be formed. N+ LDDs 56 and 58 are patterned and implanted in p-type epitaxial layer 18 by conventional techniques. Then, p+ LDDs 60 and 62 are patterned and implanted into n-well 22 by conventional techniques. As shown in Figure 3d, sidewall oxides 64 are now formed on the vertical edges of gates 40, 44, 48, and 52 as is well known in the art (e.g., CVD deposition of silicon dioxide and followed by an anisotropic etch). Following the formation of sidewall oxides 64, n+ source/drain regions 68 and 70 are patterned and implanted (e.g., 2E15 As/cm 2 at 80 keV plus 4E14 P/cm 2 at 60 keV), as shown in Figure 3d. Other dopant concentrations will be apparent to those skilled in the art. However, typically the doping implant dose will be in the range 1E15/cm 2 to 5E15/cm 2 . It will also be apparent to those skilled in the art that an implant of either arsenic or phosphorous or a combination may be used. Finally, p+ source/drain regions 72 and 74 are patterned and implanted (e.g., 1.5E15 B/cm 2 ). Typically, the doping implant dose will be in the range of 1E15/cm 2 to 5E15/cm 2 and the dopant species may, for example, be boron or BF 2 . Because polysilicon layer 36, and thus gates 40, 44, 48, and 52, are at least as thick as source/drain regions 56, 58, 60, and 62, the implants forming the source/drain regions 56-62, will only partially dope the polysilicon gates 40 and 52, as shown in Figure 4a with only a small amount of dopant reaching the bottom surface of the polisilicon. The high energy implant and anneal performed prior to the gate etch combined with the source/drain implants, completely dope polysilicon gates 44 and 48, as shown in Figure 4b. The light doping at the bottom of high voltage NMOS and PMOS gates 40 and 52, enables the electric field to penetrate into the bottom of the polysilicon gate. Figure 5a shows an electric potential band diagram for a high voltage PMOS transistor, such as transistor 88, having a lightly doped gate according to the invention. The electric field extends from the oxide into the polysilicon gate making the effective oxide thickness greater than the actual oxide thickness. Figure 5b shows a band diagram for a low voltage PMOS transistor, such as transistor 84, having a highly doped gate. The electric field will not penetrate into the low voltage areas because of the heavy doping at the bottom of the low voltage gates. The electric field remains within the oxide region. Figures 5a and 5b are for the case of PMOS transistors with p-type poly. The case for NMOS transistors with n-type poly would have similar band diagrams but with the opposite polarity. An experiment was performed using three different gate oxide thicknesses: 45 A, 55 A, and 65 A (10Å = 1nm). Table I shows the Capacitance-Voltage test results for the experiment. The nomenclature PG12 indicates that these wafers had extra patterned implants of 7E15-P/cm2 over NMOS and 7E15-B/cm2 over PMOS. These implants are in addition to the source/drain implants that are patterned after gate etch (and go into both the poly gate and the source/drains). The NMOS source/drain implant is 2E15-As/cm2 @ 80keV plus 4E14-P/cm2 @ 60keV. The PMOS source/drain implant is 1.5E15-B/cm2 @ 20keV. The polysilicon thickness is 2500A in all cases. The column tOX ACC indicates the effective oxide thickness derived from the maximum capacitance at |3.5V| in accumulation. The column tOX INV gives the effective oxide thickness derived from the maximum inversion capacitance measured by Quasi-Static techniques at |3.5|V in inversion. Note for wafer 09 without the extra implant, that the effective NMOS capacitance in inversion is 73A in comparison to 59.7A in accumulation. Even larger differences can be obtained between cases "S/D" and S/D+PG12" by increasing the poly thickness or decreasing the S/D dose. ANALYSIS OF C-V DATA FOR LOT-9628 NMOS PMOS Wafer/implant ACC tOX volts VFB INV tOX CONC. SUBST tOX ACC VFB volts tOX INV SUB CONC. 03 S/D 40.8A -1.03V 53 A 3.5E17 42.5A +1.04V 51 A 4.9E17/cm3 04 S/D+PG12 41.1 -1.00 43 3.6E17 42.9 +1.13 47.8 4.2E17 09 S/D 59.7 -1.07 73 3.5E17 60.4 +1.05 69.6 4.9E17 10 S/D+PG12 60.0 -1.03 63 3.6E17 60.7 +1.15 66.6 4.5E17 14 S/D 52.3 -1.06 61.7 3.6E17 53.3 +1.03 60.0 4.6E17 16 S/D+PG12 51.7 -1.00 52.7 3.6E17 52.6 +1.12 60.6 4.2E17 Figures 6a and 6b show the capacitance-voltage and gate current characteristics for 120A gate oxide. The effect can be most clearly seen in Figure 6a for NMOS. For this NMOS structure, both the poly gate and the transistor channel are p-type. Therefore, a negative gate bias causes both the gate and the channel to be accumulated. The capacitance decreases in accumulation as the gate voltage is decreased towards zero because the majority carrier concentration is decreasing as the channel surface potential moves towards depletion: the change in capacitance is due to a change in the Debye length with surface majority carrier concentration. Near VG=+8V, a secondary minimum in capacitance occurs when the bottom of the poly is fully depleted. At VG=+10V, both the poly surface and the channel surface are inverted. In the region between +1V and +8V, the channel is inverted and the poly is partially depleted. The slow rise in capacitance as the gate voltage VG is decreased from +8V to +1V is due to the change in depletion depth of the poly bottom surface and this change is slow because the poly is much more heavily doped than the channel. The preferred embodiments of the invention take advantage of the fact of lower capacitance in the VG=+1V to VG=+8V region. Although the NMOS gate oxide thickness is often measured electrically in accumulation at -5V, an NMOS transistor normally operates only between the range VG=0V and VG=5V (for a 5V supply). Thus, in actual operation, the transconductance and gate capacitance are set by the capacitance with the channel in depletion of inversion. It is important to note that the poly surface in this region is partially accumulated or partially depleted with the capacitance slowly rising: thus, the capacitance is low because the Debye length or depletion depth in the poly is comparable with the gate oxide thickness - minority carriers in the poly are not involved. Figures 7a, 7b, 8a, and 8b show the Capacitance-Voltage characteristics corresponding to wafers 14 and 15 in Table I. Each figure shows the measured high frequency C-V, the theoretical high frequency C-V, and the measured Quasi-Static C-V. Figure 7a shows the PMOS C-V with source/drain implant alone. Figure 7b shows the NMOS C-V with source/drain implant alone. Figure 8a shows the PMOS C-V with extra dopant and Figure 8b shows the NMOS C-V with extra dopant. Note the difference is capacitance in channel inversion with and without the added 7E15/cm2 implant for NMOS and PMOS. For example, the difference between tOX INV for the source/drain implant alone, as shown in Figure 7a and tOX INV for wafer 16 with the added implant, as shown in Figure 8b is on the order of 10 A. A device which does not fall within the scope of the claims is shown in Figure 9. Device 110 is identical to device 10 except that all transistors are high voltage. Thus, the fabrication of device 110 is identical to that described above for device 10 except that the implants and anneals done prior to the polysilicon etch are omitted. Device 110 has a substrate 114, which may for example be p++, and an optional epitaxial layer 118, which may be p-type. N-well 122 is located in epitaxial layer 118. Field insulating regions 126 may comprise oxide and serve to isolate transistors 176, 180, 184, and 188 from each other. Gate oxide regions 132 and sidewall oxides 164 insulate polysilicon gates 140, 144, 148, and 152. NMOS transistors 176 and 180 both operate at a higher voltage and are located in p-type epitaxial layer 118. NMOS transistor 176 comprises gate 140 and source/drain regions 168. Transistor 176 may also comprise lightly doped drains (LDDs) 156. NMOS transistor 180 comprises gate 144 and source/drain regions 170. Transistor 180 may also comprise LDDs 158. PMOS transistors 184 and 188 are located in n-well 122. PMOS transistor 184 comprises a gate 148, source/drain regions 172, and optionally LDDs 160. High voltage PMOS transistor 188 comprises a gate 152, source/drain regions 174, and optionally LDDs 162. An important aspect of the preferred embodiments is that the poly thickness may be chosen thicker than in the usual design. Generally for CMOS using patterned n+/p+ poly gates, the poly gate thickness is adjusted relative to the depth of the source/drain junctions such that the source/drain implants also sufficiently dope the bottom of the poly gate but the poly thickness should not be so thin that the source/drain implants penetrate the gate oxide and compensate the channel. In this invention, the poly gate thickness may be chosen relatively thick so that those transistors for high voltage operation that are doped by the source/drain implants alone will not be heavily doped on the bottom surface of the poly gate: in this way, the electric field will penetrate into the poly. Since the patterned implants to dope the poly for the low voltage transistors are implanted before gate etch, higher implant energies and doses can be used to adequately dope the bottom of the poly without regard to the requirements for the source/drain doping profiles. It should be noted that if BiNMOS input/output drivers are used, then only the added implant for the NMOS transistors would be performed. If the input/output circuits are NMOS only, then obviously only the added pattern for NMOS would be performed. It should also be noted that the low voltage transistors can be built with an unpatterned n+ implant thus producing low-voltage buried channel PMOS transistors using n + poly rather than p + poly and high voltage surface channel transistors. The foregoing description of the electric field penetration into the bottom of the polysilicon gate has been explained by reference to Figures 5, 6, 7, and 8 (a and b) which describe the situation over the active channel of the transistor. It should be noted that the electric field will also penetrate into the lightly doped polysilicon over the edges of the gate where the gate passes over the LDD or source drain. In fact, the penetration of the electric field into the lightly doped polysilicon is even greater than over the active channel because the electric field current cannot penetrate into the silicon underneath the gate oxide because the LDD or S/D is more heavily doped. The method described can be used to form not only a semiconductor device, but also an integrated circuit at the surface of a semiconductor body.

Claims: A method for forming a semiconductor device having a semiconductor body (12), comprising the steps of: a. forming a gate oxide (32) over a surface of said semiconductor body (12); b. depositing a layer (36) of polysilicon over said gate oxide (32); c. masking said polysilicon layer (36) overlying a first NMOS region and exposing a first portion of said polysilicon layer (36) overlying a second NMOS transistor region, said transistors (80) having a lower operating voltage than transistors (76) in said first NMOS region; d. implanting said first portion of said polysilicon layer (36) with an n-type dopant at a first implant dose; e. etching said polysilicon layer (36) and said gate oxide (32) to form at least one first NMOS gate (40) in said first NMOS region and at least one second NMOS gate (44) in said second NMOS region; and f. implanting first and second source/drain regions (68) on opposite sides of said first NMOS gate (40), third and fourth source/drain regions (70) on opposite sides of said second NMOS gate (44), said first NMOS gate (40) and said second NMOS gate (44) with said n-type dopant at a second implant dose. The method of claim 1 further comprising the steps of: a. patterning said polysilicon layer (36) to expose a second PMOS region after implanting said second NMOS region; b. implanting said second PMOS region of said polysilicon layer (36) with a p-type dopant at a third dopant level prior to etching said polysilicon layer (36), wherein said etching said polysilicon layer step also forms at least one second PMOS gate (48) in said second PMOS region and at least one first PMOS gate (52) in a first PMOS region; the transistors in said second PMOS region having a lower operating voltage than the transistors in said first PMOS region and c. implanting fifth and sixth source/drain regions (74) on opposite sides of said first PMOS gate (52), seventh and eighth source/drain regions (72) on opposite sides of said second PMOS gate (48), said first PMOS gate (52) and said second PMOS gate (48) with said p-type dopant at a fourth dopant level. A method as claimed in claim 1 or claim 2 and further comprising the step of annealing said semiconductor body (12) after said step of implanting said second region. A method as claimed in any of the preceding claims and wherein said dopant implant doses for said first transistor is in the range of 1E15/cm 2 to 5E15/cm 2 and said dopant implant doses for said second transistor is in the range of 1E15/cm 2 to 1E16/cm 2 .

Inventors: CHAPMAN RICHARD A, CHAPMAN, RICHARD A.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1994
Main classification: H01L27
IPC codes: H01L21, H01L27, H01L29